1. Field of the Invention
The present invention relates to a solid state image picking-up device, and more particularly, to a solid state image picking-up device in which defects (white dots or lines) can be prevented from being produced on an image picked up by the image picking-up device when high energy photons, such as in a radial ray, are input to the image picking-up device.
2. Description of Related Art
FIG. 1 is a schematic plan view showing a conventional solid state image picking-up device such as a charge coupled device (CCD). The CCD includes a photodiode 101 which performs photo-electronic conversion and a MOS transistor gate 105 which is provided for the photodiode 101 for reading out a block of signal charge stored in the photodiode 101. For each of a plurality of pixels, vertical CCD registers 102 are provided for the photodiodes 101 in a vertical direction, for transferring the blocks of signal charge read out by the MOS transistor gates 105 in a vertical direction, a horizontal CCD register 103 for transferring the blocks of signal charge transferred by the vertical CCD registers 102 in a horizontal direction, and an output section 4 for converting the blocks of signal charge transferred by the horizontal CCD register 103 into a voltage signal and outputting it.
Next, the operation of the conventional CCD will be described below. Light is inputted to the photodiode 101 through an optical system (not shown) and is subjected to photo-electron conversion to produce a block of signal charge therein. The produced block of signal charge is accumulated in the photodiode 101. After the accumulation of signal charge in the photodiode 101 is performed for a predetermined time period (e.g., 1/60th of a second), the MOS transistor gates 105 are activated for the respective photodiodes 101 at one time so that the blocks of signal charge are read out from the photodiodes 101 to the vertical CCD registers 102, as indicated by an arrow mark A. Each of the vertical CCD registers 102 transfers one block of signal charge to the horizontal CCD register 103 for every horizontal scanning period as indicated by an arrow mark B. Thus, the horizontal CCD register 103 receives the blocks of signal charge for one horizontal line from the vertical CCD registers 102 in parallel for each horizontal scanning period. Then, the horizontal CCD register 103 outputs the received blocks of signal charge to the output section 104 over the one horizontal scanning period. The output section 104 converts the blocks of signal charge for the one horizontal scanning period into an image signal for one horizontal line and outputs it.
FIG. 2 is a cross sectional view of the CCD of FIG. 1 taken along the line II--II. In FIG. 2, the CCD includes a p-type well region 121 formed on an n-type semiconductor (silicon) substrate 111. There are provided for each pixel in the well region 121 an n-type impurity region 113 for a photodiode, a p-type high density impurity region 125 which is provided on the surface of n-type impurity region 113, an n-type impurity region 115 for a CCD channel, and a p-type high density impurity region 123 for functioning as an isolating region of elements from each other. The CCD also includes a silicon oxide film 131 as a first insulating film formed on the well region 121, a charge transfer electrode 141 made of polycrystalline silicon and interposed in the first insulating film 131, an insulating film 136 as a second insulating film composed of a silicon oxide film, a light shielding film 151 provided above the charge transfer electrode 141 via the insulating film 136 to prevent incident light from inputting to the n-type impurity region 115 as the CCD channel, an insulting film 138 as a third insulating film formed of silicon oxide film (SiO.sub.2). The CCD further includes a resin film 161 for flattening the surface of the third insulating film 138 and a microlens 171 formed on the resin film 161 for focusing the incident light in the n-type impurity region 113 for the photodiode. In this CCD, a vertical CCD register is constituted of the n-type impurity region 115 and the charge transfer electrode 141.
In the CCD having the structure shown in FIG. 2, the photodiode having a p-n junction is constituted of the n-type impurity region 113 and a p-type well region 121. Light incident to the n-type impurity region 113 along a path 181 is photo-electric converted to produce electron-hole pairs there. The produced electrons are stored in the n-type impurity region 113. The charge transfer electrode 141, the n-type impurity regions 113 and 115 constitutes a MOS transistor gate in which the electrode 141 and the n-type impurity regions 113 and 115 act as a gate electrode, source and drain regions, respectively. If the charge transfer electrode 141 is applied with a voltage pulse of 10 to 15 V, the electrons stored in the photodiode are read out to the CCD channel. Then, by sequentially applying voltage pulses having different phases to the charge transfer electrodes, the electrons are transferred in a direction perpendicular to the figure plane.
As described above, in the conventional CCD, a silicon oxide film is typically used as the second insulating film 136. Also, a film of metal such as aluminium (Al), tungsten (W) or a film of silicide such as MoSi and WSi is used as the light shielding film 151. The light shielding film is used as a wiring film at the peripheral of the device although it is not shown in the figure. When a portion having a large step is on the surface of second insulating film 136, breaking of the light shielding film and wiring film is often caused, and a non-etched portion remains after etching. As a result, the manufacturing yield of the device is reduced. A technique for preventing this is proposed in, for example, Japanese Laid Open patent Disclosure (JP-A-Hei4-218965) in which a BPSG film is used as the second insulating film 136 as shown in FIG. 5 of the reference. The BPSG film is a film in which phosphorus and boron are contained in SiO.sub.2 and is softened and fluidized if it is heated to 800.degree. to 900.degree. C. Therefore, a gentle surface can be accomplished by filling the large step portion with the BPSG film and then by performing heat treatment. As a result, the breaking of the film such as the wiring film and shielding film can be prevented. That is, in the conventional CCD, the BPSG film is provided under the light shielding film.
The BPSG film can be readily fluidized as it contains more phosphorus and boron, so that the BPSG film surface can be made flat, gentle and smooth. However, if the BPSG film contains too much phosphorus and boron, phosphorus and boron diffuse toward the first insulating film 131 provided under the BPSG film, pass through the first insulating film 131, and reach the Si substrate or the p-type high density impurity region 125. As a result, the device can become inoperable. This phenomenon depends upon the thickness of the first insulating film and the density of boron/phosphorus in the BPSG film. Therefore, in this conventional CCD, the thickness of the first insulating film must be chosen to be 0.1 to 0.3 .mu.m, and the density of boron/phosphorus in the BPSG film must be chosen to be in a range of 2 to 5%.
In a conventional solid state image picking-up device, an image defect such as white dots and lines on the picked up image is caused when high energy photons such as an X-ray and a radiation ray is irradiated to the device. This is because a large dark current is generated at particular pixels. As an example of circumstances in which the high energy photons are inputted to the device, there could be considered a monitor camera of an apparatus such as a X-ray imaging apparatus and radiation ray therapeutic apparatus in a medical field, a monitor camera in a nuclear energy field, and an observation camera in a space technology field. Since such an image defect degrades the image quality, there is the need for a solid state image picking-up device which has a resistance to high energy photons such as a radiation ray and an X-ray.